Electronic devices containing active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes (OLEDs), organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices containing active organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.
An organic light-emitting device has a substrate carrying an anode, a cathode and an organic light-emitting layer containing a light-emitting material between the anode and cathode.
In operation, holes are injected into the device through the anode and electrons are injected through the cathode. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of the light-emitting material combine to form an exciton that releases its energy as light.
Cathodes include a single layer of metal such as aluminium, a bilayer of calcium and aluminium as disclosed in WO 98/10621; and a bilayer of a layer of an alkali or alkali earth compound and a layer of aluminium as disclosed in L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152 (1997).
An electron-transporting or electron-injecting layer may be provided between the cathode and the light-emitting layer.
Bao et al, “Use of a 1H-Benzoimidazole Derivative as an n-Type Dopant and To Enable Air-Stable Solution-Processed n-Channel Organic Thin-Film Transistors” J. Am. Chem. Soc. 2010, 132, 8852-8853 discloses doping of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by mixing (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) with PCBM and activating the N-DMBI by heating.
US 2014/070178 discloses an OLED having a cathode disposed on a substrate and an electron-transporting layer formed by thermal treatment of an electron-transporting material and N-DMBI. It is disclosed that a radical formed on thermal treatment of N-DMBI may be a n-dopant.
U.S. Pat. No. 8,920,944 discloses n-dopant precursors for doping organic semiconductive materials.